Indicator or signal lamps have traditionally been built around standard resistance-type incandescent bulbs. Unfortunately such bulbs are relatively inefficient, consuming considerable electrical energy while producing limited actual light. In fact the principal output of such lamps is heat.
It is known to use fluorescent bulbs, which are many times more efficient than incandescent ones, for general lighting purposes on a boat. Such lamps require alternating-current energization, and, since it is necessary to convert the on-board direct-current voltage anyhow, it has been found that conversion to high frequencies causes the bulbs to work even more efficiently than they do with standard 50 Hz or 60 Hz energization. Tests have shown that the amount of light emitted for a given current consumption increases up to about 35 kHz, and then remains the same. Thus it is known to provide a converter circuit right in the lamp that converts the standard 24-volt direct current on-board power to high-frequency alternating current.
Use of such fluorescent fixtures as indicator lamps has been ruled out because their bulbs generate considerable radio-frequency interference, right up into the megaHerz range. In fact when the light output of such lamps is being boosted by increasing the frequency of the driving voltage, the output of r-f interference increases greatly, so that starting at a certain level it effectively becomes impossible to comply with mandated levels of r-f interference suppression.
Another disadvantage of fluorescent lamps for marine use is that they do not have the desired visibility curve. A navigational light, such as a masthead light, should be plainly visible from in front of itself over an arc of 215.degree., with approximately equal brilliance over this range, and in the next 10.degree. should drop from visibility. A fluorescent lamp is normally of such structure that it constitutes several light sources, rather than what is effectively a point source created by the small filament of an incandescent bulb, and does not have the desired visibility-falloff characteristic.